This invention relates to a method and apparatus for assuring the accurate reproduction of data transmitted or received in a variety of applications including, but not limited to, use in a facsimile transmission system.
In facsimile applications, it is important to accurately sense, transmit, receive and reproduce black/white information on a document located at one location and reproduce that information at another location so as to generate an accurate and faithful facsimile. However, a substantial amount of detail which was present at the transmitting location may be lost such that the reproduction at the receiving location is less than faithful. Difficulties in achieving a faithful reproduction or facsimile are greatly compounded by the limited bandwidth available for a typical facsimile transmission over the telephone network where the information rate is high.
Video enhancement techniques have been employed in facsimile apparatus. Such a technique is disclosed in U.S. Pat. No. 3,868,477--Katzman wherein a black and white threshold detector is disclosed in combination with positive and negative peak detectors to achieve black/white transitions even when the signal lies on one side of the threshold. Black and white threshold detectors have also been utilized to enhance data as disclosed in U.S. Pat. No. 3,723,649--Pitegoff et al and U.S. Pat. No. 3,413,412--Townsend.
In the above-described Katzman patent, the peak detectors include a capacitor which stores a past black or white peak value. The current value of the video signal is then compared against the stored past peak value. When the current value differs from the stored past black or white peak value by a predetermined amount, a black/white transition in the circuit output occurs. One difficulty with the Katzman circuit is that if the time constant and the rate of charging of the storage capacitor is too slow, the detector will not be able to respond to rapid black/white changes. It would be desirable to faithfully transmit rapidly changing, i.e., high speed black/white data even over lines which are bandwidth limited.
Moreover, the data enhancement technique disclosed in the Katzman patent while reproducing black and white does not provide for the enhancement of a gray scale. In other words, a change in signal level on one side of the threshold is reproduced as black or white but there is no reproduction of various shades of gray or means by which a gray effect can be achieved.